Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers, and the power amplifier may often be the primary consumer of power in a personal audio device, and thus, may have the greatest effect on the battery life of the personal audio device. In devices having a linear power amplifier for the output stage, power is wasted during low signal level outputs, because the voltage drop across the active output transistor plus the output voltage will be equal to the constant power supply rail voltage. Therefore, amplifier topologies such as Class-G and Class-H are desirable for reducing the voltage drop across the output transistor(s) and thereby reducing the power wasted in dissipation by the output transistor(s). In such topologies, power consumption is reduced by employing a power supply, typically a charge pump power supply, which has selectable modes of operation based on an amplitude of an audio output signal of the power amplifier, wherein each of the selectable modes provides a different bi-polar supply voltage across power supply rails of the power amplifier.
While such topologies are more energy efficient than predecessor topologies, such topologies may still waste power. For example, when an audio output signal is of a positive polarity, load current to the audio output is typically delivered only from a positive supply rail of the power amplifier, and a quiescent power may be wasted in maintaining a voltage on the negative supply rail by the power supply. Similarly, when an audio output signal is of a negative polarity, load current to the audio output is typically delivered only from a negative supply rail of the power amplifier, and a quiescent power may be wasted in maintaining a voltage on the positive supply rail by the power supply.
Therefore, it would be desirable to provide a power amplifier circuit for a consumer audio device that has improved efficiency and reduced power dissipation, while maintaining a specified full-signal output level capability.